1. Field of the Invention
The present invention relates to an image heating apparatus for heating a toner image on a sheet.
2. Description of the Related Art
An apparatus discussed in Japanese Patent Application Laid-Open No. 8-152807 uses a thermal head as a heating source of a fixing apparatus (an image heating apparatus). More specifically, the thermal head includes a number of heat generating elements which are arranged side by side along a longitudinal direction of a fixing member (a heat rotatable member). The thermal head turns on the heating elements in an area (a sheet passing portion) where a recording material (a sheet) passes through and turns off the heating elements in an area (a non sheet passing portion) where the recording material does not pass through. Thus, when such a heating mechanism is employed, in a case where image formation is performed on a recording material having a small width size, no electrical power needs to be supplied to the heating elements corresponding to the non sheet passing portion.
However, when image formation is performed on a recording material having a small width size (i.e., a small size recording material) and then the image formation is continuously performed on a recording material which is wider than the previous one (i.e., a large size recording material), there is a possibility that a temperature of the fixing member in an area which was the non sheet passing portion at the time of the image formation on the small size recording material is too low if compared with a temperature of an area which was the sheet passing portion.
Therefore, it is necessary to optimize temperature distribution in the longitudinal direction of the fixing member in a period from when the image formation on the small size recording material is finished to when the image formation on the large size recording material is started. In other words, in this case, a preparatory operation needs to be performed for selectively heating an area which was the non sheet passing portion of the fixing member.
Therefore, during the preparatory operation, the image formation on the large size recording material cannot be started.